(1) Field of the Invention
This invention relates to the local manipulation of electromagnetic fields, and more particularly, but not exclusively, to the use of radiation manipulating devices to allow RF (radio frequency) tags to be mounted on materials which would otherwise impede their use.
(2) Description of the Art
RF tags are widely used for the identification and tracking of items, particularly for articles in a shop or warehouse environment. One commonly experienced disadvantage with such tags is that if directly placed on a metal surface their read range is decreased to unacceptable levels and more typically the tag cannot be read or interrogated. This is because a propagating-wave RF tag uses an integral antenna to receive the incident radiation: the antenna's dimensions and geometry dictate the frequency at which it resonates, and hence the frequency of operation of the tag (typically 866 MHz, or 915 MHz, with 860-960 MHz being the approved range for a UHF (ultra-high frequency) range tag and 2.4-2.5 GHz or 5.8 GHz for a microwave-range tag). When the tag is placed near or in direct contact with a metallic surface, the tag's conductive antenna interacts with that surface, and hence its resonant properties are degraded or—more typically—negated. Therefore the tracking of metal articles such as cages or containers is very difficult to achieve with UHF RF tags and so other more expensive location systems have to be employed, such as GPS.
UHF RFID tags also experience similar problems when applied to any surfaces which interact with RF waves such as, certain types of glass and surfaces which possess significant water content, such as, for example, certain types of wood with a high water or sap content. Problems will also be encountered when tagging materials which contain/house water such as, for example, water bottles, drinks cans or human bodies etc.
This problem is particularly true of passive tags; that is tags which have no integrated power source and which rely on incident energy for operation. However, semi passive and active tags, which employ a power source such as an onboard battery also suffer detrimental effects on account of this problem.
One way around this problem is to place a foam spacer, or mounting between the RF tag and the surface, preventing interaction of the antenna and the surface. With currently-available systems the foam spacer needs to be at least 10-15 mm thick in order to physically distance the RF tag from the surface by a sufficient amount. Clearly, a spacer of this thickness is impractical for many applications and is prone to being accidentally knocked and damaged.
Other methods have involved providing unique patterned antennas which have been designed to impedance match a particular RF tag with a particular environment.